Field of the Invention
The present invention relates to an automatic transmission for a vehicle, and more particularly, to a planetary gear train of an automatic transmission for a vehicle, which is capable of implementing nine forward speeds using a minimum number of components, improving power transmission performance and fuel efficiency by increasing a span of a gear shift ratio, and ensuring linearity of ratios between gear shift stages.
Description of Related Art
Recent increases in oil prices are triggering stiff competition among auto-makers in enhancing fuel consumption of a vehicle.
Therefore, research is being conducted on engines in terms of reducing weight and improving fuel efficiency by down-sizing, and research is also being conducted to ensure both drivability and competitiveness for maximizing fuel efficiency by implementing an automatic transmission with multiple speed stages.
However, in the case of the automatic transmission, the number of internal components increases as the number of gear shift stages is increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.
Accordingly, development of a planetary gear train which may bring about maximum efficiency with a small number of components is important in order to increase a fuel efficiency enhancement effect through the multiple speed stages.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, in the case of the recent 8-speed automatic transmission, since a span of a shift ratio is maintained to a level of 6.5 to 7.5, the 8-speed automatic transmission does not have a great effect of improving the fuel efficiency.
In addition, in the case in which the span of the shift ratio of the 8-speed automatic transmission is set to a level of 9.0 or more, since linearity of an interstage ratio of a shift stage may not be secured, running efficiency of the engine and drivability of the vehicle are degraded.
Therefore, there is a need to develop a high efficient automatic transmission of at least 9-speed stages.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.